This invention relates to a method for quenching heated glass sheets with air nozzles to strengthen them.
More particularly, the present invention relates to a method of quenching glass sheets wherein a glass sheet moving through a quenching section is blown in the same portions throughout the quenching section with air from nozzles and is moved while being rapidly cooled in the same portions so as to set up the differentials of residual strains on the glass surface.
A glass sheet used as a windshield of an automobile is heated in a heating furnace, is then conveyed to a quenching section, is blown with air and is rapidly cooled to be a tempered glass sheet. A compression strain is developed on the rapidly cooled glass surface and a tension strain corresponding to the compression strain is developed in the interior portion which chills gradually, with the strength of the glass being determined by the differential rates of cooling between the surface and the interior of the glass body.
Nowadays, a standard tends to be set for the purpose of avoiding the danger when a side glass or rear glass of an automobile is broken. This standard is to regulate whether or not there are long pieces (called splines) over the length of 6 cm. produced when the glass breaks, and the number of broken pieces. According to the already set British Standard No. BS 5282 "Road Running Vehicle Safety Glass," it is required that the number of broken pieces per unit area of 5.times.5 cm. of a glass sheet of a thickness no more than 4 mm. should be in a range of 40 to 400. Where a glass sheet is rapidly cooled and tempered to generate a compression strain by quenching the glass surface, if this strain is made large, splines will be eliminated but the number of broken pieces per unit area will increase. If the strain is made to be below a certain value in order to keep the number of broken pieces within an allowable upper limit, contrary to the above, splines will be produced. It has recently been required to solve this technical problem and it is desired to prevent the generation of splines while keeping the number of broken pieces within the allowable range defined by the standard.
The present inventors have made the present invention in order to meet the above requirement. Particularly, by obtaining a knowledge that, in quenching step, if the glass sheet is partially quenched on the surface and the strain differentials are set up on the surface, the generation of splines will be able to be prevented while keeping the number of broken pieces within a predetermined range.